A continuing demand for improved telephone services in both voice and data telecommunications has resulted in various evolutionary trends, one of which is referred to as an integrated services digital network (ISDN) that features a wide range of voice and data applications in the same network. The ISDN concept emphasizes versatility which is apparent in the variety of applications that are supported thereby. This trend towards ISDN systems is especially noteworthy for its digital end-to-end connectivity that is achieved by extending digital technology to the user terminals of a telephone system.
Although ISDN versatility is extensive, substantial installation and operating costs of an ISDN system may be prohibitive for small business telephone subscribers. Moreover, because such costs are significant, users currently operating less sophisticated and possibly older equipment may be similarly prohibited from trading up to experience the broad capabilities of ISDN.
Key telephone systems have been popular with small business users because of increased connectivity features that are available to them at reasonable cost. A limitation of the system, however, resides in its need for switches and cabling because all available telephone features are effected through key switches. Consequently, at some point of growth development in a small business, inevitable limits will be imposed on its communications capability because of inadequate switching services. This would then necessitate a complete changeover to an ISDN system. However, connectivity between a key telephone system and a central office or local telephone exchange having ISDN capability would occur by way of plain ordinary telephone system (POTS) lines which are limited by a narrow bandpass characteristic. The amount of information that can be shared between the central office and the key telephone system is therefore correspondingly limited.